The invention is based on a fuel injection pump for internal combustion engines as defined hereinafter.
With fuel injection pumps and in particular unit fuel injectors of this kind, a greater degree of freedom in terms of open- and closed-loop control interventions into the entire course of injection is attained than can be done with injection pumps of a distributor or in-line type. However, this higher degree of freedom entails a higher operating pressure, particularly when used in high-speed direct injection engines. Moreover, the camshaft of the engine is used to drive the pump piston, making it possible to economize on additional drive losses an making higher drive forces available for the possibly necessary higher injection pressures. On the other hand, these high drive forces dictate suitably dimensioned restoring springs, which require a great amount of space.
The high drive forces also bring strong shear forces to bear on the drive tappet. The point of engagement of these drive forces on the drive tappet must therefore be located as low as possible, to avoid canting of the tappet in the guide tube.
In a known unit fuel injector of this generic type (U.S. Pat. No. 4,452,574), a ball tappet bolt is therefore interposed between the drive element and the drive tappet; it engages the bottom of the cupshaped tappet insert. This embodiment has the further advantage that because of the ball tappet bolt, an additional degree of freedom is obtained, by means of which tolerances in length can be compensated for.
On the other hand, however, this known unit fuel injector has the disadvantage that the tappet diameter is relatively large, so that the remaining clearance for the restoring spring and tappet guidance is small. Because the pump piston is suspended inside the intermediate sleeve, which at the same time is used to secure the ball of the loss prevention device from falling out, the intermediate sleeve must fit around the tappet insert, which increases the minimum diameter of the drive tappet. As a result, however, the contact point of the tappet insert on the pump piston is then also located relatively high up, so that the pump guide must extend high up as well. Because of the strong driving forces, however, a guide sleeve for the drive tappet that extends far downward is equally necessary, and it therefore has a larger diameter in order to be capable of fitting over the piston guide. The overall result is a relatively large total diameter for the drive tappet.
Another disadvantage of this unit fuel injector is that the pump piston must be pulled out as well in order to remove the loss prevention device.